Hybrid electric vehicles supplement the internal combustion engine power with one or more electric machines and selectively allocate demanded power to the engine and the electric machines during a drive cycle. These arrangements improve fuel economy and reduce emissions as compared to conventional power systems that solely rely on an internal combustion engine as a power source. The ability to maximize these benefits is a function of the allocation of the power split between the engine and the electric machines utilized to meet driver torque demand. For example, hybrid electric vehicles allow the engine to be shut down when the electric machines and stored energy are capable of meeting driver torque demand, and the engine can be automatically restarted when the available stored energy and/or torque demand requires operation of the engine. Hybrid electric vehicles also allow regenerative energy to be captured and stored or utilized immediately to power the electric machines to drive the vehicle. Furthermore, the engine and electric machines can be controlled to operate at points of highest efficiency during a drive cycle to minimize the use of fuel while meeting output torque requirements.
One problem encountered during a drive cycle of a hybrid electric vehicle involves transitioning from an electric drive mode in which driver torque demand is met solely by the electric machines to a hybrid drive mode in which driver torque demand is met by both the engine and the electric machines. While the electric machines can be employed to start the engine during this transition, the driver can feel a reduced torque at the wheels as torque from the electric machines is allocated to start the engine, or if the transmission is disengaged or slipped relative to the electric machines during engine starting. If the torque from the electric machines remains engaged to the drive wheels during starting of the engine, the driver may feel an increased torque at the wheels as the engine speed increases to meet the speed of the electric machines. Also, the vehicle can jerk during transition from the electric drive mode to the hybrid drive mode, which can be unsettling for the driver and impart undesirable loading to the drive train components. While various solutions have been proposed to address this problem, there remains a need for further technological advancements in this area.